fmt: change the overflow test for large numbers in verbs

The old one was inferior.

Fixes #10695.

Change-Id: Ia7fb88c9ceb1b10197b77a54f729865385288d98
Reviewed-on: https://go-review.googlesource.com/9709Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/fmt/fmt_test.go

@@ -538,6 +538,7 @@ var fmtTests = []struct {
 	{"%T", nil, "<nil>"},
 	{"%-1", 100, "%!(NOVERB)%!(EXTRA int=100)"},
 	{"%017091901790959340919092959340919017929593813360", 0, "%!(NOVERB)%!(EXTRA int=0)"},
+	{"%184467440737095516170v", 0, "%!(NOVERB)%!(EXTRA int=0)"},
 
 	// The "<nil>" show up because maps are printed by
 	// first obtaining a list of keys and then looking up

src/fmt/print.go

@@ -291,10 +291,12 @@ func parsenum(s string, start, end int) (num int, isnum bool, newi int) {
 		return 0, false, end
 	}
 	for newi = start; newi < end && '0' <= s[newi] && s[newi] <= '9'; newi++ {
-		num = num*10 + int(s[newi]-'0')
-		if num < 0 {
+		const maxInt32 = 1<<31 - 1 // 31 bits is plenty for a width.
+		max := maxInt32/10 - 1
+		if num > max {
 			return 0, false, end // Overflow; crazy long number most likely.
 		}
+		num = num*10 + int(s[newi]-'0')
 		isnum = true
 	}
 	return